| Message ID | 20190416134522.17540-22-ldufour@linux.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Speculative page faults | expand |
On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote: > This allows to search for a VMA structure without holding the mmap_sem. > > The search is repeated while the mm seqlock is changing and until we found > a valid VMA. > > While under the RCU protection, a reference is taken on the VMA, so the > caller must call put_vma() once it not more need the VMA structure. > > At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a > reference is taken to the VMA by calling get_vma(). > > When removing a VMA from the MM RB tree, the VMA is not release immediately > but at the end of the RCU grace period through vm_rcu_put(). This ensures > that the VMA remains allocated until the end the RCU grace period. > > Since the vm_file pointer, if valid, is released in put_vma(), there is no > guarantee that the file pointer will be valid on the returned VMA. > > Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com> Minor comments about comment (i love recursion :)) see below. Reviewed-by: Jérôme Glisse <jglisse@redhat.com> > --- > include/linux/mm_types.h | 1 + > mm/internal.h | 5 ++- > mm/mmap.c | 76 ++++++++++++++++++++++++++++++++++++++-- > 3 files changed, 78 insertions(+), 4 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 6a6159e11a3f..9af6694cb95d 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -287,6 +287,7 @@ struct vm_area_struct { > > #ifdef CONFIG_SPECULATIVE_PAGE_FAULT > atomic_t vm_ref_count; > + struct rcu_head vm_rcu; > #endif > struct rb_node vm_rb; > > diff --git a/mm/internal.h b/mm/internal.h > index 302382bed406..1e368e4afe3c 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -55,7 +55,10 @@ static inline void put_vma(struct vm_area_struct *vma) > __free_vma(vma); > } > > -#else > +extern struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, > + unsigned long addr); > + > +#else /* CONFIG_SPECULATIVE_PAGE_FAULT */ > > static inline void get_vma(struct vm_area_struct *vma) > { > diff --git a/mm/mmap.c b/mm/mmap.c > index c106440dcae7..34bf261dc2c8 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -179,6 +179,18 @@ static inline void mm_write_sequnlock(struct mm_struct *mm) > { > write_sequnlock(&mm->mm_seq); > } > + > +static void __vm_rcu_put(struct rcu_head *head) > +{ > + struct vm_area_struct *vma = container_of(head, struct vm_area_struct, > + vm_rcu); > + put_vma(vma); > +} > +static void vm_rcu_put(struct vm_area_struct *vma) > +{ > + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); > + call_rcu(&vma->vm_rcu, __vm_rcu_put); > +} > #else > static inline void mm_write_seqlock(struct mm_struct *mm) > { > @@ -190,6 +202,8 @@ static inline void mm_write_sequnlock(struct mm_struct *mm) > > void __free_vma(struct vm_area_struct *vma) > { > + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) > + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); > mpol_put(vma_policy(vma)); > vm_area_free(vma); > } > @@ -197,11 +211,24 @@ void __free_vma(struct vm_area_struct *vma) > /* > * Close a vm structure and free it, returning the next. > */ > -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) > +static struct vm_area_struct *__remove_vma(struct vm_area_struct *vma) > { > struct vm_area_struct *next = vma->vm_next; > > might_sleep(); > + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) && > + !RB_EMPTY_NODE(&vma->vm_rb)) { > + /* > + * If the VMA is still linked in the RB tree, we must release > + * that reference by calling put_vma(). > + * This should only happen when called from exit_mmap(). > + * We forcely clear the node to satisfy the chec in ^ Typo: chec -> check > + * __free_vma(). This is safe since the RB tree is not walked > + * anymore. > + */ > + RB_CLEAR_NODE(&vma->vm_rb); > + put_vma(vma); > + } > if (vma->vm_ops && vma->vm_ops->close) > vma->vm_ops->close(vma); > if (vma->vm_file) > @@ -211,6 +238,13 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) > return next; > } > > +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) > +{ > + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) > + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); Adding a comment here explaining the BUG_ON so people can understand what is wrong if that happens. For instance: /* * remove_vma() should be call only once a vma have been remove from the rbtree * at which point the vma->vm_rb is an empty node. The exception is when vmas * are destroy through exit_mmap() in which case we do not bother updating the * rbtree (see comment in __remove_vma()). */ > + return __remove_vma(vma); > +} > + > static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags, > struct list_head *uf); > SYSCALL_DEFINE1(brk, unsigned long, brk) > @@ -475,7 +509,7 @@ static inline void vma_rb_insert(struct vm_area_struct *vma, > > /* All rb_subtree_gap values must be consistent prior to insertion */ > validate_mm_rb(root, NULL); > - > + get_vma(vma); > rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks); > } > > @@ -491,6 +525,14 @@ static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm) > mm_write_seqlock(mm); > rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks); > mm_write_sequnlock(mm); /* wmb */ > +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT > + /* > + * Ensure the removal is complete before clearing the node. > + * Matched by vma_has_changed()/handle_speculative_fault(). > + */ > + RB_CLEAR_NODE(&vma->vm_rb); > + vm_rcu_put(vma); > +#endif > } > > static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma, > @@ -2331,6 +2373,34 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr) > > EXPORT_SYMBOL(find_vma); > > +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT > +/* > + * Like find_vma() but under the protection of RCU and the mm sequence counter. > + * The vma returned has to be relaesed by the caller through the call to > + * put_vma() > + */ > +struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, unsigned long addr) > +{ > + struct vm_area_struct *vma = NULL; > + unsigned int seq; > + > + do { > + if (vma) > + put_vma(vma); > + > + seq = read_seqbegin(&mm->mm_seq); > + > + rcu_read_lock(); > + vma = find_vma(mm, addr); > + if (vma) > + get_vma(vma); > + rcu_read_unlock(); > + } while (read_seqretry(&mm->mm_seq, seq)); > + > + return vma; > +} > +#endif > + > /* > * Same as find_vma, but also return a pointer to the previous VMA in *pprev. > */ > @@ -3231,7 +3301,7 @@ void exit_mmap(struct mm_struct *mm) > while (vma) { > if (vma->vm_flags & VM_ACCOUNT) > nr_accounted += vma_pages(vma); > - vma = remove_vma(vma); > + vma = __remove_vma(vma); > } > vm_unacct_memory(nr_accounted); > } > -- > 2.21.0 >
On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote: > This allows to search for a VMA structure without holding the mmap_sem. > > The search is repeated while the mm seqlock is changing and until we found > a valid VMA. > > While under the RCU protection, a reference is taken on the VMA, so the > caller must call put_vma() once it not more need the VMA structure. > > At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a > reference is taken to the VMA by calling get_vma(). > > When removing a VMA from the MM RB tree, the VMA is not release immediately > but at the end of the RCU grace period through vm_rcu_put(). This ensures > that the VMA remains allocated until the end the RCU grace period. > > Since the vm_file pointer, if valid, is released in put_vma(), there is no > guarantee that the file pointer will be valid on the returned VMA. What I'm missing here, and in the previous patch introducing the refcount (also see refcount_t), is _why_ we need the refcount thing at all. My original plan was to use SRCU, which at the time was not complete enough so I abused/hacked preemptible RCU, but that is no longer the case, SRCU has all the required bits and pieces. Also; the initial motivation was prefaulting large VMAs and the contention on mmap was killing things; but similarly, the contention on the refcount (I did try that) killed things just the same. So I'm really sad to see the refcount return; and without any apparent justification.
On Tue, 23 Apr 2019, Peter Zijlstra wrote: >Also; the initial motivation was prefaulting large VMAs and the >contention on mmap was killing things; but similarly, the contention on >the refcount (I did try that) killed things just the same. Right, this is just like what can happen with per-vma locking. Thanks, Davidlohr
Le 23/04/2019 à 11:27, Peter Zijlstra a écrit : > On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote: >> This allows to search for a VMA structure without holding the mmap_sem. >> >> The search is repeated while the mm seqlock is changing and until we found >> a valid VMA. >> >> While under the RCU protection, a reference is taken on the VMA, so the >> caller must call put_vma() once it not more need the VMA structure. >> >> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a >> reference is taken to the VMA by calling get_vma(). >> >> When removing a VMA from the MM RB tree, the VMA is not release immediately >> but at the end of the RCU grace period through vm_rcu_put(). This ensures >> that the VMA remains allocated until the end the RCU grace period. >> >> Since the vm_file pointer, if valid, is released in put_vma(), there is no >> guarantee that the file pointer will be valid on the returned VMA. > > What I'm missing here, and in the previous patch introducing the > refcount (also see refcount_t), is _why_ we need the refcount thing at > all. The need for the VMA's refcount is to ensure that the VMA will remain until the end of the SPF handler. This is a consequence of the use of RCU instead of SRCU to protect the RB tree. I was not aware of the refcount_t type, it would be better here to avoid wrapping. > My original plan was to use SRCU, which at the time was not complete > enough so I abused/hacked preemptible RCU, but that is no longer the > case, SRCU has all the required bits and pieces. When I did test using SRCU it was involving a lot a scheduling to run the SRCU callback mechanism. In some workload the impact on the perfomance was significant [1]. I can't see this overhead using RCU. > > Also; the initial motivation was prefaulting large VMAs and the > contention on mmap was killing things; but similarly, the contention on > the refcount (I did try that) killed things just the same. Doing prefaulting should be doable, I'll try to think further about that. Regarding the refcount, I should I missed something, this is an atomic counter, so there should not be contention on it but cache exclusivity, not ideal I agree but I can't see what else to use here. > So I'm really sad to see the refcount return; and without any apparent > justification. I'm not opposed to use another mechanism here, but SRCU didn't show good performance with some workload, and I can't see how to use RCU without a reference counter here. So please, advise. Thanks, Laurent. [1] https://lore.kernel.org/linux-mm/7ca80231-fe02-a3a7-84bc-ce81690ea051@intel.com/
Le 22/04/2019 à 22:57, Jerome Glisse a écrit : > On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote: >> This allows to search for a VMA structure without holding the mmap_sem. >> >> The search is repeated while the mm seqlock is changing and until we found >> a valid VMA. >> >> While under the RCU protection, a reference is taken on the VMA, so the >> caller must call put_vma() once it not more need the VMA structure. >> >> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a >> reference is taken to the VMA by calling get_vma(). >> >> When removing a VMA from the MM RB tree, the VMA is not release immediately >> but at the end of the RCU grace period through vm_rcu_put(). This ensures >> that the VMA remains allocated until the end the RCU grace period. >> >> Since the vm_file pointer, if valid, is released in put_vma(), there is no >> guarantee that the file pointer will be valid on the returned VMA. >> >> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com> > > Minor comments about comment (i love recursion :)) see below. > > Reviewed-by: Jérôme Glisse <jglisse@redhat.com> Thanks Jérôme, see my comments to your comments on my comments below ;) >> --- >> include/linux/mm_types.h | 1 + >> mm/internal.h | 5 ++- >> mm/mmap.c | 76 ++++++++++++++++++++++++++++++++++++++-- >> 3 files changed, 78 insertions(+), 4 deletions(-) >> >> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h >> index 6a6159e11a3f..9af6694cb95d 100644 >> --- a/include/linux/mm_types.h >> +++ b/include/linux/mm_types.h >> @@ -287,6 +287,7 @@ struct vm_area_struct { >> >> #ifdef CONFIG_SPECULATIVE_PAGE_FAULT >> atomic_t vm_ref_count; >> + struct rcu_head vm_rcu; >> #endif >> struct rb_node vm_rb; >> >> diff --git a/mm/internal.h b/mm/internal.h >> index 302382bed406..1e368e4afe3c 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -55,7 +55,10 @@ static inline void put_vma(struct vm_area_struct *vma) >> __free_vma(vma); >> } >> >> -#else >> +extern struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, >> + unsigned long addr); >> + >> +#else /* CONFIG_SPECULATIVE_PAGE_FAULT */ >> >> static inline void get_vma(struct vm_area_struct *vma) >> { >> diff --git a/mm/mmap.c b/mm/mmap.c >> index c106440dcae7..34bf261dc2c8 100644 >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -179,6 +179,18 @@ static inline void mm_write_sequnlock(struct mm_struct *mm) >> { >> write_sequnlock(&mm->mm_seq); >> } >> + >> +static void __vm_rcu_put(struct rcu_head *head) >> +{ >> + struct vm_area_struct *vma = container_of(head, struct vm_area_struct, >> + vm_rcu); >> + put_vma(vma); >> +} >> +static void vm_rcu_put(struct vm_area_struct *vma) >> +{ >> + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); >> + call_rcu(&vma->vm_rcu, __vm_rcu_put); >> +} >> #else >> static inline void mm_write_seqlock(struct mm_struct *mm) >> { >> @@ -190,6 +202,8 @@ static inline void mm_write_sequnlock(struct mm_struct *mm) >> >> void __free_vma(struct vm_area_struct *vma) >> { >> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) >> + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); >> mpol_put(vma_policy(vma)); >> vm_area_free(vma); >> } >> @@ -197,11 +211,24 @@ void __free_vma(struct vm_area_struct *vma) >> /* >> * Close a vm structure and free it, returning the next. >> */ >> -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) >> +static struct vm_area_struct *__remove_vma(struct vm_area_struct *vma) >> { >> struct vm_area_struct *next = vma->vm_next; >> >> might_sleep(); >> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) && >> + !RB_EMPTY_NODE(&vma->vm_rb)) { >> + /* >> + * If the VMA is still linked in the RB tree, we must release >> + * that reference by calling put_vma(). >> + * This should only happen when called from exit_mmap(). >> + * We forcely clear the node to satisfy the chec in > ^ > Typo: chec -> check Yep > >> + * __free_vma(). This is safe since the RB tree is not walked >> + * anymore. >> + */ >> + RB_CLEAR_NODE(&vma->vm_rb); >> + put_vma(vma); >> + } >> if (vma->vm_ops && vma->vm_ops->close) >> vma->vm_ops->close(vma); >> if (vma->vm_file) >> @@ -211,6 +238,13 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) >> return next; >> } >> >> +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) >> +{ >> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) >> + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); > > Adding a comment here explaining the BUG_ON so people can understand > what is wrong if that happens. For instance: > > /* > * remove_vma() should be call only once a vma have been remove from the rbtree > * at which point the vma->vm_rb is an empty node. The exception is when vmas > * are destroy through exit_mmap() in which case we do not bother updating the > * rbtree (see comment in __remove_vma()). > */ I agree ! >> + return __remove_vma(vma); >> +} >> + >> static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags, >> struct list_head *uf); >> SYSCALL_DEFINE1(brk, unsigned long, brk) >> @@ -475,7 +509,7 @@ static inline void vma_rb_insert(struct vm_area_struct *vma, >> >> /* All rb_subtree_gap values must be consistent prior to insertion */ >> validate_mm_rb(root, NULL); >> - >> + get_vma(vma); >> rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks); >> } >> >> @@ -491,6 +525,14 @@ static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm) >> mm_write_seqlock(mm); >> rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks); >> mm_write_sequnlock(mm); /* wmb */ >> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT >> + /* >> + * Ensure the removal is complete before clearing the node. >> + * Matched by vma_has_changed()/handle_speculative_fault(). >> + */ >> + RB_CLEAR_NODE(&vma->vm_rb); >> + vm_rcu_put(vma); >> +#endif >> } >> >> static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma, >> @@ -2331,6 +2373,34 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr) >> >> EXPORT_SYMBOL(find_vma); >> >> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT >> +/* >> + * Like find_vma() but under the protection of RCU and the mm sequence counter. >> + * The vma returned has to be relaesed by the caller through the call to >> + * put_vma() >> + */ >> +struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, unsigned long addr) >> +{ >> + struct vm_area_struct *vma = NULL; >> + unsigned int seq; >> + >> + do { >> + if (vma) >> + put_vma(vma); >> + >> + seq = read_seqbegin(&mm->mm_seq); >> + >> + rcu_read_lock(); >> + vma = find_vma(mm, addr); >> + if (vma) >> + get_vma(vma); >> + rcu_read_unlock(); >> + } while (read_seqretry(&mm->mm_seq, seq)); >> + >> + return vma; >> +} >> +#endif >> + >> /* >> * Same as find_vma, but also return a pointer to the previous VMA in *pprev. >> */ >> @@ -3231,7 +3301,7 @@ void exit_mmap(struct mm_struct *mm) >> while (vma) { >> if (vma->vm_flags & VM_ACCOUNT) >> nr_accounted += vma_pages(vma); >> - vma = remove_vma(vma); >> + vma = __remove_vma(vma); >> } >> vm_unacct_memory(nr_accounted); >> } >> -- >> 2.21.0 >> >
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 6a6159e11a3f..9af6694cb95d 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -287,6 +287,7 @@ struct vm_area_struct { #ifdef CONFIG_SPECULATIVE_PAGE_FAULT atomic_t vm_ref_count; + struct rcu_head vm_rcu; #endif struct rb_node vm_rb; diff --git a/mm/internal.h b/mm/internal.h index 302382bed406..1e368e4afe3c 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -55,7 +55,10 @@ static inline void put_vma(struct vm_area_struct *vma) __free_vma(vma); } -#else +extern struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, + unsigned long addr); + +#else /* CONFIG_SPECULATIVE_PAGE_FAULT */ static inline void get_vma(struct vm_area_struct *vma) { diff --git a/mm/mmap.c b/mm/mmap.c index c106440dcae7..34bf261dc2c8 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -179,6 +179,18 @@ static inline void mm_write_sequnlock(struct mm_struct *mm) { write_sequnlock(&mm->mm_seq); } + +static void __vm_rcu_put(struct rcu_head *head) +{ + struct vm_area_struct *vma = container_of(head, struct vm_area_struct, + vm_rcu); + put_vma(vma); +} +static void vm_rcu_put(struct vm_area_struct *vma) +{ + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); + call_rcu(&vma->vm_rcu, __vm_rcu_put); +} #else static inline void mm_write_seqlock(struct mm_struct *mm) { @@ -190,6 +202,8 @@ static inline void mm_write_sequnlock(struct mm_struct *mm) void __free_vma(struct vm_area_struct *vma) { + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); mpol_put(vma_policy(vma)); vm_area_free(vma); } @@ -197,11 +211,24 @@ void __free_vma(struct vm_area_struct *vma) /* * Close a vm structure and free it, returning the next. */ -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) +static struct vm_area_struct *__remove_vma(struct vm_area_struct *vma) { struct vm_area_struct *next = vma->vm_next; might_sleep(); + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) && + !RB_EMPTY_NODE(&vma->vm_rb)) { + /* + * If the VMA is still linked in the RB tree, we must release + * that reference by calling put_vma(). + * This should only happen when called from exit_mmap(). + * We forcely clear the node to satisfy the chec in + * __free_vma(). This is safe since the RB tree is not walked + * anymore. + */ + RB_CLEAR_NODE(&vma->vm_rb); + put_vma(vma); + } if (vma->vm_ops && vma->vm_ops->close) vma->vm_ops->close(vma); if (vma->vm_file) @@ -211,6 +238,13 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) return next; } +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) +{ + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma); + return __remove_vma(vma); +} + static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags, struct list_head *uf); SYSCALL_DEFINE1(brk, unsigned long, brk) @@ -475,7 +509,7 @@ static inline void vma_rb_insert(struct vm_area_struct *vma, /* All rb_subtree_gap values must be consistent prior to insertion */ validate_mm_rb(root, NULL); - + get_vma(vma); rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks); } @@ -491,6 +525,14 @@ static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm) mm_write_seqlock(mm); rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks); mm_write_sequnlock(mm); /* wmb */ +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT + /* + * Ensure the removal is complete before clearing the node. + * Matched by vma_has_changed()/handle_speculative_fault(). + */ + RB_CLEAR_NODE(&vma->vm_rb); + vm_rcu_put(vma); +#endif } static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma, @@ -2331,6 +2373,34 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr) EXPORT_SYMBOL(find_vma); +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT +/* + * Like find_vma() but under the protection of RCU and the mm sequence counter. + * The vma returned has to be relaesed by the caller through the call to + * put_vma() + */ +struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, unsigned long addr) +{ + struct vm_area_struct *vma = NULL; + unsigned int seq; + + do { + if (vma) + put_vma(vma); + + seq = read_seqbegin(&mm->mm_seq); + + rcu_read_lock(); + vma = find_vma(mm, addr); + if (vma) + get_vma(vma); + rcu_read_unlock(); + } while (read_seqretry(&mm->mm_seq, seq)); + + return vma; +} +#endif + /* * Same as find_vma, but also return a pointer to the previous VMA in *pprev. */ @@ -3231,7 +3301,7 @@ void exit_mmap(struct mm_struct *mm) while (vma) { if (vma->vm_flags & VM_ACCOUNT) nr_accounted += vma_pages(vma); - vma = remove_vma(vma); + vma = __remove_vma(vma); } vm_unacct_memory(nr_accounted); }
This allows to search for a VMA structure without holding the mmap_sem. The search is repeated while the mm seqlock is changing and until we found a valid VMA. While under the RCU protection, a reference is taken on the VMA, so the caller must call put_vma() once it not more need the VMA structure. At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a reference is taken to the VMA by calling get_vma(). When removing a VMA from the MM RB tree, the VMA is not release immediately but at the end of the RCU grace period through vm_rcu_put(). This ensures that the VMA remains allocated until the end the RCU grace period. Since the vm_file pointer, if valid, is released in put_vma(), there is no guarantee that the file pointer will be valid on the returned VMA. Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com> --- include/linux/mm_types.h | 1 + mm/internal.h | 5 ++- mm/mmap.c | 76 ++++++++++++++++++++++++++++++++++++++-- 3 files changed, 78 insertions(+), 4 deletions(-)